C++面向对象程序设计双语教程（第3版）课件 ch07Polymorphism and Virtual Functions

PolymorphismandVirtualFunctionsC++面向对象程序设计双语教程(第3版)Chapter701PolymorphismThekeyideabehindOOPispolymorphism.PolymorphismisderivedfromaGreekwordmeaning“manyforms”.Wespeakthetypesrelatedbyinheritanceaspolymorphictypes，becauseinmanycaseswecanusethe"manyforms³ofaderivedorbasetypeinterchangeably.IntroductiontoPolymorphismPolymorphismInprogramminglanguages，polymorphismmeans

that

some

codesoroperationsor

objectsbehavedifferentlyindifferentcontexts.Forexample，the+operatorinC++implementtheadditionoperationwithdifferentdataordifferenttypes.IntroductiontoPolymorphismPolymorphismPolymorphismisakeyfeatureofobject-orientedprogrammingthatallowsthevaluesofdifferentdata

typestobehandledbyusingauniforminterface.多态性是一个面向对象程序设计的关键特性，它允许使用统一的接口处理不同数据类型。PolymorphismThepurposeofpolymorphismistoimplementastyleofprogrammingcalledmessage-passingintheliterature，inwhichtheobjectsofvarioustypesdefineacommoninterfaceofoperationsforusers.PolymorphismTypically，polymorphismoccurswhenthereisaclasshierarchyinwhichtheclassesarerelatedbyinheritance.PolymorphismBindingWhenaC++programisexecuted，itexecutessequentially，beginningatthetopof

functionmain.Whenafunctionealliseneountered，thepointofexecutionjumpstotheinitial

pointofthefunctionbeingcalled.HowdoestheCPUknowtodothis？PolymorphismWhenaprogramiscompiled，thecompilerconvertseachstatementinyourC++programintooneormorelinesofmachinelanguage.Eachlineofmachinelanguageisgivenitsownuniquesequentialaddress.Thisisnodifferentforfunctionswhenafunctionisencountered，itisconvertedintomachinelanguageandgiventhenextavailableaddress.Thus，eachfunction

endsupwithauniquemachinelanguageaddress.PolymorphismBindingisaprocessthatisusedtoconvertidentifiers

intomachinelanguageaddresses.Inotherwords，bindingisanassociation，suchasbetweenidentifiers

andoperations.Therearetwotypesofbindingaccordingtothetimeatwhichabindingtakesplace，i.e.staticbindingand

dynamicbinding.绑定是一种用于将标识符（如变量和函数名）转换成机器语言地址的过程。换句话说，绑定是一种关联，例如在标识符（变量或函数名）和操作（类型或特定函数体）之间的关联。根据绑定发生的时间，有两种类型的绑定，即静态绑定和动态绑定。PolymorphismStaticBindingBydefault，Ct+matchesafunctioncallwiththecorrectfunctiondefinitionatcompiletime.Thisiscalledstatiebinding.Thiskindofbindingisalsoknownaseompile-timebinding.Forexample，theoperatoroverloadingandthefunctionoverloadingmentionedinthepreviouschaptersarethatthecompilerassociatestheoperatorfunctionsandoverloadedfunctionswiththeirdefinitionsatcompile-time.PolymorphismEachfunctionhasauniqueaddress.Thus，whenthecompilerencountersafunctioncall，itreplacesthefunctioncallwithamachinelanguageinstructionthattellstheCPUtojumptotheaddressofthefunction.PolymorphismWhenthecompilermatchesafunctioncallwiththecorreetfunctiondefinitionatrun-time，thisiscalleddynamiebinding.Thecodeassociatedwiththeproceduredoesnotknowuntiltheprogramisexecuted.Thiskindofbindingisalsoknownasrun-timebinding.Youcandeclare

afunctionwiththevirtualkeywordifyouwantthecompilertousedynamicbindingforthatspecificfunction.DynamicBindingPolymorphism02VirtualFunctionsAvirtualfunctionisamemberfunctionofthebaseclassandisredefinedbythederivedclass.Thecompilerwillguaranteethecorrectcorrespondencebetweenobjectsandthefunctionsappliedtothem.Avirtualfunctioniscreatedusingthekeywordvirtualwhichprecedesthenameofthefunction.VirtualFunctionsForexample，supposeagraphicsprogramincludesseveraldifferentshapes：acubeandacuboid，andsoon，asshowninFigure7-1.VirtualFunctionsToenablethiskindofbehavior，wedeclarefunctionvolumeinthebaseclassasavirtualfunction，andweoverridefunctionvolumeineachofthederivedclassestocalculatethevolumeofappropriateshape.VirtualFunctionsWhenaderivedclassinheritstheclasscontainingthevirtualfunction，ithastheabilityto

redefinethevirtualfunctions.Avirtualfunctionhasdifferentfunctionalityinthederivedclass.Thevirtualfunetionimplementsthephilosophyofoneinterfaceandmultiplemethods.VirtualFunctionsVirtualfunetionscanbeaccessedbyusingabaseclasspointer.Apointertothebaseclasscanbecreated.Abaseclasspointercancontaintheaddressofthederivedobjectasthederivedobjectcontainsthesubsetofabaseclassobject.VirtualFunctionsEveryderivedclassisalsoabaseclass.Whenabaseclasspointercontainstheaddressofthederivedclassobject，atruntime，whichversion

ofthevirtualfunctionbeingcalleddependsonthetypeofobjectcontainedbythepointer.

Hereisaprogramwhichillustratestheworkingofvirtualfunctions.VirtualFunctionsOnceafunctionisdeclaredasvirtual，itremainsvirtualallthewaydowntheinheritancehierarchy

fromthatpoint，evenifthatthefunctionisnotexplicitlydeclaredasvirtualwhenaclassoverrides

it.Ifthememberfunctioninthederivedclassisvirtual，thekeywordvirtualcanbeomittedinthe

derivedclass.Butthekeywordvirtualcannotbeomittedinthebaseclass.VirtualFunctionsExtensibilityIfthevolumefunetionisdefinedasvirtualinthebaseclass，youcanaddasmanynewclasstypesasyouwantwithoutchangingthefnfunction.Inawell-designedOOPprogram，mostorallyourfunetionswillfollowthemodeloffunctionfnandcommunicateonlywiththebase-classinterface.VirtualFunctionsExtensibilitySuchaprogramisextensiblebecauseyoucanaddnewfunctionalitybyinheritingnewdatatypesfromthecommonbaseclass.Thefunetionsthatmanipulatethebase

classinterfacewillnotneedtobechangedatalltoaccommodatethenewclasses.HereistheShapeexamplewithmorevirtualfunctionsandanumberofnewclasses，allofwhichworkcorrectlywiththeold，unchangedfnfunction.VirtualFunctionsTheextensibilityofclasshierarchymentionedinExample7-3isshowninFigure7-2.VirtualFunctionsHowcandynamiebindinghappenbyvirtualfunctions？Alltheworkgoesonbehindthescenesbythecompiler，whichinstallsthenecessarydynamicbindingmechanismwhenyouaskitto（youareaskedtocreatevirtualfunctions）.PrincipleofVirtualFunctionsVirtualFunctionsBecauseprogrammersoftenbenefitfromunderstandingthemechanismofvirtualfunctionsinC++，thissectionwillelaborateonthe

waythecompilerimplementsthismechanism，Thevirtualkeywordtellsthecompilerthatitshouldnotperformstaticbinding.PrincipleofVirtualFunctionsVirtualFunctionsIneachclasswithvirtualfunctions，itsecretlyplacesapointer，calledthevpointer（abbreviatedasVPTR），whichpoints

totheVTABLEforthatobject，asshowninFigure7-3.VirtualFunctionsVirtualDestructorsOnethingrecommendedfortheclasswithpointermembervariablesisthattheclassshouldhavethedestructor（seeSection4.6.2）.Thedestructorisautomaticallyexecutedwhentheclassobjectgoesoutofthescope.Then，iftheobjectcreatesdynamicobjects，thedestructorcanbedesignedtodeallocatethestorageforthem.VirtualFunctionsThebaseclassdestructorshouldalwaysbevirtual.Supposeyouusethedeleteoperatorwithabaseclasspointertoaderivedclassobjecttodestroythederivedclassobject.Ifthebaseclassdestructorisnotvirtual，butlikeanormalmemberfunction，thenthedeleteoperator

callsthedestruetorforthebaseclass，butnotthedestructorforthederivedclass.VirtualFunctionsIfnoneofthedestructorshasanythingimportanttodo（likedeallocatingmemoryobtainedwith

delete），thenvirtualdestructorsaren'tnecessary.But，ingeneral，toensurethatderivedclassobjects

aredestroyedproperly，youshouldmakevirtualthedestruetorsinallbaseclasses.VirtualFunctionsWehaveintroducedfunctionoverloadingandfunctionoverridingintheprevioussections.Thesetermsaresimilar，andtheydosimilarthings.Whenyouoverrideamemberfunetionoftheclass，youcreateamemberfunetioninaderivedclasswiththesamenameasafunetioninthebaseclassandthesamesignature.FunctionOverloadingandFunctionOverridingVirtualFunctionsIfwedefineafunctionasvirtualinthebaseclass，thefunctionwiththesame

name

andsamesignaturebutnokeywordvirtualinthederivedclassisalsothevirtualfunctionbecauseofoverriding.VirtualFunctions03AbstractBaseClassesOfteninadesign，youwantthebaseclasstopresentonlyaninterfaceforitsderivedclasses.Thatis，youdon'tallowanyonetocreateanobjectofthebaseclass，butonlywanttoupcasttoitsothatitsinterfacecanbeused.Thisisaccomplishedbymakingthatclassabstraet.AbstractBaseClassesAbstractbaseclassesactastheexpressionsofgeneralconceptsfromwhichmorespecific

classescanbederived.Ifanabstractbaseclasscannotbeinstantiated，itexistsextensivelyforinheritanceandit

mustbeinherited.AbstractBaseClassesTherearescenariosinwhichitisusefultodefineaclassthatisnotintendedtobeinstantiatedbecausesuchclassesnormallyareusedasbase-classesininheritance

hierarchies.AbstractBaseClassesThisclassmustbeinherited.Thisclassismostlyusedasabaseclass.Youcannotcreatean

objectofanabstractelasstype；however，youcanusepointersandreferencestoabstractclasstypes.AbstractBaseClassesAclassthatcontainsatleastonepurevirtualfunctionisconsideredasanabstractclass.Theclassesderivedfromtheabstractclassmustimplementthepurevirtualfunctionorthey，too，areabstractclasses.AbstractBaseClassesApurevirtualfunctionisafunctionwhichcontainsnodefinitioninthebaseclass.Youdeclareapurevirtualfunctionbyusingapurespecifier（=0）inthedeclarationofavirtualmemberfunctionintheclassdeclaration.纯虚函数是一个在基类中没有定义的函数。在类声明时使用pure说明符（=0）将一个虚成员函数声明为纯虚函数。AbstractBaseClassesAclasswhichcontainsoneormorepurevirtualfunctioniscalledanabstractbaseclass.Abstractbase

classcsactasexpressionsofgeneralconceptsfromwhichmorespecificclassescanbederived.包含一个或多个纯虚函数的类称为抽象基类。抽象基类作为通用概念的表示，从中可以导出更多具体的类。AbstractBaseClassesThestatementinLine6tellsthecompilertoreserveaslotforafunetionintheVTABLE，butnottoputanaddressinthatparticularslot.Evenifonlyonefunctioninaclassisdeclaredaspurevirtual，theVTABLEisincomplete.AbstractBaseClassesIftheVTABLEforaclassisincomplete，whatisthecompilersupposedtodowhen

someonetriestomakeanobjectofthatclass？Itcannotsafelycreateanobjectofanabstractclass，soyougetanerrormessagefromthecompiler.Thus，thecompilerguaranteesthepurityoftheabstractclass.Bymakingaclassabstract，youcanensurethatthecl